fix: clean build error output #78

Problem

When user code has a compile error, default mcpp build output is noisy and confusing:

1. Ninja progress lines such as [1/9] OBJ obj/main.o are shown.
2. The generated command exposes long env LD_LIBRARY_PATH=... prefixes.
3. The same ninja/compiler output can be printed twice.

The local reproduction and proposed implementation plan are documented in .agents/docs/2026-05-26-build-error-output-optimization-plan.md.

Scope

• Keep default build errors focused on compiler diagnostics.
• Preserve full details under --verbose.
• Move tool runtime environment handling out of visible ninja command lines.
• Add regression coverage for the default build-error output.